Parallelogram lifts, wherein a series of parallel legs are pivotally mounted between a base and a support platform, are well known for lifting small vehicles such as motorcycles, all-terrain vehicles, snowmobiles, and the like. These lifts, which are exemplified by U.S. Pat. No. 5,518,224 to Anderson, may be actuated from a lowered state wherein the support platform, pivoting legs, and base are all adjacently situated in substantially parallel relation to a raised state wherein the platform is spaced in parallel relation from the base by the pivoting legs.
These prior lifts are subject to several disadvantages. Initially, they tend to be configured so that substantial force must be initially applied to the lifts to begin elevating the support platform with respect to the base. The Anderson patent illustrates a typical arrangement wherein the actuator used to push the pivoting legs upward acts on an attachment point on the pivoting legs adjacent the base of the lift. If the kinematics of this arrangement are analyzed, it is seen that this arrangement requires very high actuating force to be applied at the attachment point in order to begin pivoting the legs. In effect, if the pivoting legs are viewed as levers which pivot at a fulcrum at the base, this arrangement situates the actuating force nearer the fulcrum and thereby requires an actuating force of greater magnitude. As a result, lifting is initially difficult and it then becomes easier as lifting progresses. When a user applies a constant actuating force to the lift, this tends to cause the lift to initially elevate slowly and then speed up as the support platform nears its fully raised position. This can be problematic because the sudden stop experienced when the support platform reaches its fully raised position can then upset the vehicle on the support platform. Additionally, because of the high actuation forces that need to be exerted on the attachment point on the pivoting legs, the actuating mechanism and its linkages to the attachment point must be heavily reinforced in order to avoid excessive wear and possible failure. This requires that heavier (and more expensive) components be used, or alternatively that the rated weight capacity of the lift be limited so that failure does not occur from overloading.
Further, many prior lifts use "simple" pivot arrangements wherein a pin is simply inserted in turn through the pivoting legs and then through the base, and similarly through the pivoting legs and then the support platform. The pivoting legs then pivot in planes adjacent to the pin-connected members in the base and support platform. Such arrangements, as exemplified by Anderson, exert high shear forces on the pin. As a result, these pins and their surrounding components must be reinforced (leading to greater expense) or the rated weight capacity of the lift must be decreased. Additionally, such simple pivot arrangements decrease the overall rigidity of the lift in planes perpendicular to the planes wherein the pivoting legs rotate, thereby increasing the tendency of the support platform to wobble in these perpendicular planes.
In addition, lifts such as Anderson's are difficult to properly situate beneath vehicles because they lack wheels or other components allowing them to be easily relocated on the ground. Some prior lifts bear wheels, but these are generally mounted to the lifts by extending a plate from the top of the lift's base in cantilever fashion, and then affixing the wheels beneath the plate. Since the entire weight of the lift and vehicle must be supported by the wheels, the cantilevered plate, and the adjoining section of the base, these components must be fortified to avoid failure. This is particularly true of the plate, which can sag or bend when the lift is loaded and cause the lift to "bottom out" on the ground. If the plates are severely overloaded, they can fail and cause the lift's base to fall to the floor, which can in turn cause the vehicle to be dislodged from the support platform to fall to the ground. The cantilevered arrangement can be avoided by simply mounting wheels to the bottom of the base, but this tends to increase the height of the lift to an unacceptable degree owing to the increased difficulty in fitting the lift under low vehicles.